Direct-viewing electronic storage tubes



Aug. 13, 1957 A. v. HAEFF DIRECT-VIEWING ELECTRONIC STORAGE TUBES 4 SheetsSheet 1 Original Filed Jan. l2, 1952 A. V. HAEFF DIRECT-VIEWING ELECTRONIC STORAGE TUBES 4 Sheets-Sheet 2 JMW/w- Afri/mu Aug. 13, 1957 original Filed Jan. 12, 1952 R., n @M .Ef V4 M my w w WM w 4 Sheets-Sheet 5 Aug. 13, 1957 A, v HAI-:FF

DIRECT-VIEWING ELECTRONIC STORAGE TUBES Original FledJan. l2, 1952 Aug. 13, 1957 A. v. HAEFF 2,802,966

DIRECT-VIEWING ELECTRONIC STORAGE TUBES United States Patent tlice V2,802,966 Patented Aug. 13, 1957 DIRECT-VIEWKNG ELECTRONIC STORAGE TUBES Andrew V. Haeif, Pacific Palisades, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Continuation of application Serial No. 266,137, January 12, 1952. This application August 6, 1954, Serial No. 448,258

6 Claims. (Cl. 315-12) This invention relates to direct-viewing electronic storage tubes and more particularly to direct-viewing electronic storage tubes for presenting a series of applied electrical signals as a corresponding series of visual images, each of the visual images persisting until converted into the immediately succeeding visual image by the application of the electrical signal to which the immediately succeeding visual image corresponds. A tube of this type is suitable for both color and black and white presentations of the applied signals.

This application is a continuation of the application of Andrew V. Hae entitled Direct-Viewing Electronic Storage Tubes, Serial No. 266,137, led January l2, 1952.

One of the most desirable characteristics of an indicator tube is that each element of its viewing surface emit light in proportion to the signal received for that element. An ideal tube would be the one in which each element would emit light continuously at a level determined by the received signal until the new signal, received for that element, changes the light level to a new value. The more conventional radar indicator tubes or television kinescopes, while satisfying reasonably well the requirement of proportionality of light-to-signal, do not emit light -continuously due to rapid decay of luminescence of phosphor. The elect of continuous light emission is achieved in television by frequent excitation of the screen (high frame rate) which is compatible with the persistence of vision. The frame rate must be even higher in color television in order to avoid the icker effect, as portions of the picture may be predominantly of one of three basic colors. This requires transmitting many more frames or complete pictures than required for continuity of movement. If an ideal presentation device were used wherein the light output is continuous and the new signal simply resets the level of light out v put for each element of the viewing surface, the frame repetition rate could be reduced considerably, say to 12-16 frames a second vs. 100 frames now required for reproduction of color images. This reduction in frame rate could be used either to reduce the bandwidth required for transmitting a picture of given resolution or, since for television purposes the bandwidth has been standardized at 6 mc. per channel, an ideal presentation device would permit considerable improvement in picture detail for the same total bandwidth.

This invention discloses several storage tubes in which a reasonable approximation to the performance of an ideal presentation device can be achieved. In accordance with this invention, the received signal is used to control the effective potential of each elemental area of the storage surface. Each elemental area then acts as a control grid similar to that of a triode, for example, to control the flow of electrons towards a luminescent Viewing screen. In order to approximate the performance of the ideal presentation device, the potential on each storage element should be capable of being quickly changed to a new potential corresponding to the signal voltage and this potential maintained without appreciable change during the whole frame period or until a new signal for that element is received. Of course, reasonable correspondence between the signal voltage and the light output should also be achieved. In the devices of the prior art, the relatively rapid decay of light output from an element of the viewing screen after excitation of such element contributed to the flicker difficulties but was essential for the presentation of moving pictures. A very long persistence phosphor could not be used on the screen of the known television receiving tubes because, without decay, the whole screen would saturate and assume almost uniform brightness. However, in an ideal device essentially infinite persistence can be used very effectively provided the incoming signal is able to reset the level of light output in either direction for each element of the picture in accordance with the latest available information in the latest signal received. Of course in certain cases, such as radar, for example, it may be more desirable to permit a degree of signal integration to improve visibility through noise. As will be explained more fully in the specification, this can be achieved by allowing the potential on the elements of the storage surface to decay slower after responding to the signals so that the effective potential of each element is the summation of signals received during several consecutive scans.

It is, therefore, an object of this invention to provide a storage tube device with a storage screen on which a charge distribution can be reset either in a positive or in a negative direction with each application of a signal.

Another object of this invention is to provide a direct-viewing storage tube in which a visual image having a dynamic range corresponding to a video signal and a continuous presentation which is changed by resetting the charge distribution in either direction on the storage screen with a new signal is produced, with the concomitant reduction in flicker.

A further object of this invention is to provide a direct-viewing storage tube capable of producing a presentation with dynamic range and also capable of integrating or reinforcing the visual image over several scans for increasing the signal-to-noise ratio.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings, in which several embodiments of the invention are illustrated by way of example. lt is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a deuition of the limits of the invention.

Fig. l illustrates a longitudinal sectional view of one embodiment of the direct-viewing color storage tube;

Figs. 2 and 3 illustrate a -cross-sectional view and a.

plan view, respectively, of the storage screen of the tube illustrated in Fig. l;

Fig. 4 illustrates a family of explanatory secondary emission curves;

Fig. 5 illustrates an enlarged sectional view of the storage screen with respect to the viewing screen for the tube illustrated in Fig. l;

Fig. 6 is a plot of potential levels encountered by ilood electrons for the tube illustrated in Fig. l with reference to the enlarged sectional view of Fig. 5;

Fig. 7 illustrates a longitudinal sectional view of an alternative embodiment of a storage tube;

Fig. 8 illustrates a plot of potential levels encountered by flood electrons with reference to an enlarged sec- Figfll showsvv the manner in which the writing screen charges the; storage screen for thev tube illustrated in Fig.

v"1"`ig; 12 shows a plotvv of the potential levels encountered by ood electrons with reference to an enlarged sectional view tof theV storage'A screen, writing screen and viewing screedfor an alternate mode of operation of the tube ilunrated in; Fig. 7;

"Fig..5"l=3"illustrates, a, longitudinal sectional view of, an alternative, embodiment of" the direct-viewing color stor- .uwvw f 'g e storage tube illustrated in Fig. 1 Will produceA a visi iijn'ageyin,aecordance'with` a video,signaleithenofl black ndwhite'varietyl or lin color, thelimage being reset fro o iframe to the next so that it appears continuously thus "eliminating the problem of icker. In the case of a color presentation, each color field. i's presented continuouslyfand,simultaneously, making it unnecessary to injcreasef'the' Afrarn'cfrate'in Vorder to solve the ilicker prblemz-",

',Referringtollg 1, the tube comprises an evacuated envelop'`1`0'which, in'its left portion as viewed in the Vligure, has a, Writing gun12 with appropriate electron b'e'anifdeectingfmeans 16, Vand a flood gun 14. The writingfgnfcnsists"of'a'conventional heater element 17 connectedf'by means of yconductors 18 to a source of filament Vpotential"'20,"a button-type cathode 22, intensity grid 24, aii'dben-forming-and accelerating electrodes 36, 40 `and 411. The accelerating electrode 36-is connected through lead`35to Vthepositive terminal of a source of directpoten't'ialr34. which. holds thisv'electrode at some suitable potentialwhich may-,be of the order of 1500 volts. The electrode 4'1'is also connected to the same positive potentialthrough lead 42v`vghile-- electrode 40 is connected through lead739tovay negative,l source 'of potential 38. Electrodef40-should be maintainedat a potential of approximately 30'( )volts positive Withrespect 'tothepotential of cathode 22. Thefcathode l22 is connected yto a biasing potentialsourceZ.through a signal load resistor 30 and conductor27.j Intensity grid 24 ismaintained negative respect to cathode 22 by connectingit to the cathode throughlead26, variabledirect potential source 28, and lead A,27. Variableu directgpotential source 28is bypassed forsalternating potentials by capacitor 29. Video signal is: applied at terminal.y 15 through, capacitor 19 to the cathQdeAZLand throughcapa'citor 29,-to intensity grid 24 s-asnotrtoQvary appreciably the'. flow of electronsi'n the electron beam.4 The quiescent negative potential of cathode 22 is dependent to a large extent on the characteristics of the storage screen butwill be in the neighborhood of 3000,volts with respect to ground. TheV operation and construction of the electron gun structure of this type isV well known in. theart and therefore needs `no additional description.v i i The writing gun 12 has electron beam deilccting means 16 comprising vertical deection plates 44 and horizontal deflection plates 45' shielded from each other by electrode 46 whichis maintained at the same positive potential as electrode 41 by lead 42. While electrostatic deecting means are illustrated in the figure, it is to be understood that magnetic beamdeectioncoils.may alsoA be used, in which case the horizontal and vertical deiiection plates are replacedwith appropriate lmagnetic deflection coils. Direct-current potential from source 34 is applied' to the vertical and-horizontal deflection plates, through leads 47 and isolation resistors 48, 4'9'and V50, 51, respectively, so

- 4f V that the deection plates are at a suitable positive'V potential which maybe of the orderl of 1400 volts. Ca'- Apacitors 52, 53, S4 and 55, couple resistors 48 through 51, respectively, to suitable sources of balanced scanning voltages, the circuitry for which is notillustrated in the figure. The disclosed tube is in no way restricted to a particular mode of scanning and, since suitable scanning circuits are known in the art, no detailed description of any scanning circuit is presented.

Flood gun 14 includes a conventional heater element 67 connected through a pair of conductors 66 to a source V of filament potential, a cathode 70, a beam-forming electrode 72, and an 'accelerating electrode 74. Electrode 74 extends over electrodey 72 to shield ood gun 14 from writing gun 12 and deectingmeans 16. Beam-forming electrode 72 is connected throughda lead 71 to the negative terminal of a source 68 of direct-current potential, the positive terminal of which is, connected to cathode 70. Source 68 may produce a potential of the order of -5 volts;r VThe ypotential ofjood gun cathode is dependent on 'the particular mode lof operation. An appropriate value r for the particular mode being described is -5'0 volts with respect to4 ground. This potential is applied through a-resistork 69' from a variable potential source 64. A small magnitude alternating-current potential of sawtooth waveform'of afrequency of several megacycles may be impressed on ood gun cathode 70 to eiect an'articial velocityspread to theood electrons. The magnitude of this alternating-current potentialis dependent on the uniformity of the secondary emission characteristics of the storage surface and'may be of the order of 2 Volts peakto-peak. This artificial velocity spreading potential is appliedacross resistor 69- by means of a connection to a source 73'.

The conguration of the electrodes 72 and 74, and

suclias ,toproduc'e a uniformdensity of the primary electrons inV the'planeoi a. storage screen 78 `toward lwhich this"floodY beam is directed. rThe electric eld of a.

pair-ofelectrodes 56 and 60 form the electron optics for the oodbeam.. Thesevelectrodes are maintained Vat va sutliciently positive potential to cause theflood beam electrons passing through the regions dominated by the electrodes tospread out evenly over storage screen 7S and `impinge on each portion of it at normal incidence. Electrode 5.6 is maintained lat an appropriate positive potential by means of a connection through a conductor 57 to an internjiediatey point of` a. potential source 58, the negative terminal of which isconnected to ground. The magnitude ofthe potential appliedto electrode 56 may be of the order of 150G-volts., Electrode 60ismaintain`ed at an` appropriate positiveipotentialv by a connection through a conductor 61- to thepositive terminal of potentialzsource 58, the magnitudeof4 this potential being of the order of 300D-volts.

Au collectorl electrode 62 Vis mounted withinv envelope 1Q:totherightof.electrode 60,as` viewed in. Figkg'l, and operates to` attract Vsecondary electrons liberated` from source screen, 78.f Electrode 62y is maintained atan appropriate positive potential by a connection through a lead 63 to an intermediate tap of potential source 5S. A; representativevalue for ,the voltage applied to electrode 62`is +300 volts.

. Continuingnowwith the description of the elements in the right portion of evacuated envelope 10, as viewed in Fig. 1, a storage screen 78-is`mounted-within envelope 10 in a positionfacing theelectronguns. Storage screen 78, a portion of. whichjis4 illustrated on'a large scale in Figs. '2.and 3, includes a attenedfout line metallic mesh 76 stretched ,'between; a hoop ring v.79. One suitable material for mesh 76 is stainless steel Awhich 'offers severalfadvantages such as, for'example; resistance to corrosion dur` ing the degassing process ofA the tube and suicient strength to retain a flat configuration whenstretched between hoop lring 79 used for supporting screen' 7,8. Other metals or alloys having similar properties can be used for mesh 76, one of these being nickel.

p The size of mesh 76maybe of the order of 250 wires per inch, `with the diameter of the wire being of the order of 1.5 to 2 mils. The limitsof the number of wires 'per inch and the diameter of the wire used for making mesh 76 are not especially critical. When mesh 76 consists of horizontal wires and vertical wires, the screen, prior to its additional processing which is described below, is subjected to a flattening-out process in a hydraulic press. Sucient pressure is exerted by the two llat surfaces on the mesh 76 to` compress the cross-over wire junctions so that the center lines of the vertical and horizontal wires are in substantially the same plane `as illustrated in Figs. 2 and 3.

One side of mesh 76 is coated with a dielectric material which has a very high specific resistance. The surface of the dielectric material constitutes a storage surface .80 for the tube. Suitable dielectric materials are phosphors, especially P1 (cubic-ZnzSiOtzMn) or P11 (cubic-ZnSzAg) and silicon dioxide. The process of applying a coating` of phosphor to mesh 76 includes drawing a high velocity stream of clean air through the mesh and, at the same time, spraying the mesh from an air gun at a range of approximately 0.5 inch with an emulsion of phosphor suspended together with a binder in a liquid such as butyl acetate, one suitable binder being nitro-cellulose. Storagesurface 80 is produced by holding the spray gun so as to make the particles of the dielectric follow the free stream of air and travel in a direction normal to the mesh. Should a silicon dioxide coating be desired,` it can be applied by the conventional method of evaporating silicon dioxide in layers on mesh 76.

The general configuration of storage surface 80, as shown in Fig. 2, is semi-elliptical in cross-section, it being understood, of course,` that other cross-sectional configurations, such as rectangular and triangular, may also be used. As shown in Figs. 1 and 2, storage surface 80 faces towards the electron guns and extends along lines parallel to the longitudinal axis of the tube. The thickness of storage surface 80 along these lines determines the amount of control exerted by screen 78 on the llow of `electrons through the interstices of the screen, this thickness preferably being of the order of one to two times the diameter of wires comprising mesh 76. The overall transparency of screen 78 is not critical and may vary between wide limits, such as, for example, to 70% transparency.

After the coating of dielectric material is deposited on the mesh 76, screen 78 is subjected to a baking process in an oven in order to bind the dielectric more closely to the mesh 76 and, at the same time, to solidify the binder. While the drawings illustrate screen 78 as being composed of horizontal and vertical wires, it is to be understood that the vertical wires may be eliminated, provided the rnechanical strength of the remaining wires is suflcient to retain the desired configuration.

Mesh 76 of storage screen 78 is utilized as a contrast control grid in the disclosed storage tube and is maintained at an appropriate potential so as to regulatethe iiow of primary electrons through the interstices of storage screen 78 towards a viewing screen 88 as explained later. Accordingly, mesh 76 will henceforth be referred to as` contrast control grid 76. An appropriate potential for contrast control grid 76 may be of the order of 100 volts with respect to ground and is applied from a `source 90 of direct-current potential through a lead 77 connected between ring 79 and an intermediate terminal of a source 90 of direct current potential. t n

`Viewing screen 88 is positioned within envelope 10 closely adjacent to storage screen 78, i. e. contiguous thereto, and to the right thereof, as viewed in Fig. l. Viewing screen 88 comprises a glass plate 86, a conductive transparent layer 84 deposited on the vsideohf plate .86.exz .6 posed to` the `electrn guns, and athin layer 82 of phos'- phor deposited on layer 84.. If ,thetube is to be used for color presentations, layer 82 is composed of different colored phosphor stripes, the number. and arrangement depending on the resolution desired andthe type of scanning to be used. Alternatively, conductive layer 84 may be applied over layer 82 by evaporating 'a thin film of aluminum, for example. t, i

Transparent conductive layers 4of. the type used are. known in the art by variousnames, one ofthem being known asV Nesajwhich consists of an evaporated layer of stannous chloride. The methods,ofdepositing.such

layers are known to the prior `art obviating amore detailed description. j t p l l Conductive layer 84 is maintained at ahigh positive potential so that electrons passing through the interstices of storage screen 78 will impinge on phosphor layer 82.`

This result isaccomplished by connecting conductive layer 84 to a positive terminal oftpotential source 90 `through t a lead 85. Representative values of the potential applied to layer 84 may range from `+3000 to +2000() volts with respect to ground.

The embodiment of the present invention illustrated in Fig. 1. functions generally inthe following manner. Writing gun 12 produces an electron beam which under the control of scanning means 16 scans storage screen 78 in any desired manner such as, for example, a televisiontype or plan position indicator scan. In order to approach theperformance of an ideal presentation device, each element of storage surface of screen 78, upon bombardment by the writing or signal beam from writing gun 12, should assume a Vpotential corresponding to the new signal regardless of whether theprevious potential was below or above the new one. This characteristic may be achieved by making use of the secondary emission characteristic of the dielectric material of storage surface 80.

Referring now to Fig. 4, there is shown a group of characteristic curves of `the secondary emission ratio of' dielectric material versus the electron volts of primary electrons incident on the dielectric material. The secondary emission ratio for a given material is dened as the ratio of the number of secondary emission electrons plus the number of reliected, repelled, or turned-backprimary electrons picked up by the collector electrodes to the number of primary electrons incident on the material. There are two values of energy of bombarding electrons for materials for which the maximum secondary emission ratio is greater than unity at which the secondary emission ratio is equal to one. The first value is referred to as the critical potential, Vo, and is usually on the order of from 50 to 200 volts, while the second value is sometimes referred to as the sticking potential, Vat, and is usually of the order of from 2000 to 4000 volts. The potential of the electrode collecting secondary emission electrons,

which in Fig. 1 is collector electrode 62, is maintained' sufliciently high so that even when the storage surface element is at the sticking potential the secondary electrons can be drawn away from it. In this manner, the potential of the bombarded storage element may be changed by varying the potential of cathode 22 of writing gun 12. In other words, the bombarded element of storage surface 80 will assume a potential equal to the potential of cathode 22 of writing gun 12 plus the sticking potential which remains constant. The sticking potential magnitude depends upon the characteristics of the material of which the bombarded surface is composed. Therefore, if the whole storage surface is scanned by the writing beam while the potential of writing gun cathode 22 remains com stant, the entire storage surface exposed to the 'writing beam will assume a potential equal to the sticking poten,-Y tial relative to the potential of the writing gun cathode 22.

However, if during the scan, the potential of cathode 221 is modulated by the signal voltage, then the different ele .mental areas of surface all assume the potential` corre` spending to. thednstalntaneous value. ot thecathode'poteni.

tial plusthe stickingtpotential.'

.- 11n: Vgieater 'detail,"'the process ofchargi-ngfanelementof Ystorage'.screenl781'isfair'follovvs:Assume thatdue 'to aprevious scan,rWithffthe/potential raf-.cathodev 22. equal ,to- .Y Vw, an'element-.offstorage surface was` charged' to' a potentialr Vsfasfillustrated: in Fig.`4. `During the present scan, the potential of4 the cathode 2 21ofthewriting gun' 12 has L assumed., a new value1 Vfw whichisn negative with re-V the."be"ginnin'gl?`of the bombardment Vof an element of screen? -'by.the'yvritingbeam'the electrons will be jarriving-at'the elementl'with enei'lgy greater than the stiel;- ing potential energy by the amounttVw-V'av); As ayresult' 'thesecend'ry emission vratio' willl belessthn one, `the rieti-'owof currentVl tothe element; will; be negative and the potentialof thatyelemetwill;A decrease in the negative "di're'ction'. along curve' '92ti towards the 'stable point 94a.

'corresponding to the potential V's t,

Similarly; .i'fdilrns subsequent scan. by. thsfwliting. beam, the new potential of' cathode' 22' is Vw whichis .l .positive Withfespetfavw. @Positive Charge. Wlll'Qw to the ',lernent'to increase its potential in a positive. directed, essere 'as"fsasasepoasa having mais. V"sr asV shown'infig. 4g Therefore, iffthesticking poten-v tiallenergy is thesa'me for all 'elerpentsofl storage. surmletsd ,Wrtinggbsam will. result .in establishing. a potentialdistribution over storage surface 80 correspondingM to the;instantaneousmagnitude of the Signal voltage during the'scanninglperiodt elmentsgit. takes' .fa-1w..

oodgun; llfthroughA the .iinterstices -.orl foraminafofstoragegsreenJS and contrast4 control grid).

7 which would correspond to Athe f potentialpdistribution Y ort. storage.. sur-*face Ythe 4potential' of cathode. 70 of oodgun14relat ive tothe potential ofVA contrast control ture no vrcurrent.can;.flow-through the. intersticesfandfor highlights essentially. maximum current can ow through.

If theeffectiye velocity spread of electrons from flood gun 14..isV smalh the optimum potential of `flood-gun cathodes 70jwillybetwithina-few voltsof Vthe average potential VS1; of storage surface 80.

1 Thel representative ,values*of the potentials may'besummarizedas follows:` Referring to Fig. l

Potentialfofwriting gun cathode. 22:-3000 volts relativetoground;v

Sticking Vpotential ofvs'toragei surface: StL- 3000 volts relative rewriting. gun. Cathodezz;

Potential .0f.cslster'elsettssle 62 =500 volts .relative fo'grsuud.; ,s

'Approximate potential 'ofjQod-gun cathode 70'is 50'l voltsl relative-td ground;n l

i Potential of transparentconducting layer 84=+l0,000 volts relative 4to groundxand f v Y8 The potential: of? the writing. gun cathode ZZ isf modu-y latedbya signal applied' at terminal 15, Fig.4 1 ,ftle'signa'l1A modulation being appliedl through capacitor'19- to both cathode` 2 2` and control .grid 24 of writing` gun. 1'2?V by`v means'of' conductors 27'and 26- and bypass capacitor 29.

The average'value ofthewriting beamvv current can be? adjusted tofaf desired value by meansof'variable intensity; 'grid biasing source 28' dependingon the desired mode of operation.'- Forth'e normal modeof operation of the storage tube of theV present invention theywriting" beam current will be suciently high so that even for the's strongest signalsW thel total .charge delivered'toan element;v

' .T fidi will'be-su'lcient toraise the-'potential of the element by-f an amount equal to the'signal voltage. v However, ifsignal integration is desired, lthe writing beam currentwll be such that thel signal voltageisl greater than soV that full-charging willoccuifonly after several.' scansofi'thewriting beam: l

yIn. order to prevent' the electronsl fromsil'ood gun 14v from discharging the elements-of the storage surface 80;

thepotential' ofi flood gun cathode 70 isVA held aty a'valu'e more positive thanthe maximum potential'oftany'element on storagesurface 80; The flood electrons--will'be able to'v penetrate Athrough th'e Ainters'tices of the screen 78 in spiteof the fact that an element on storage `surface 80 maybe at amore negative potential' than thepotential'of holding gunfcahode` 70 if Vcontrast control grid 76 is maintained ata' potential somewhatV above lthe potential of ilood gun cathode 70.

Referring new togFig; 5; `there is illustrated an enlarged cross-sectional view-of storage screen 78v and Viewing screen 88 Witha pair ofcenter lines 96 for `a pair of wire elements o'contrast control grid 7 6 and theassociated` dielectric material-of storage surface 80, and a pair of center lines98 for the interstices Vof storage screen 78.

Fig. 6 shovvseaA plot of potential distribution along lines 96"and''981,;.as-det`ned in- Fig. 5; for the three`-situations illustrated in Eig. 4 Whereinthe writing gun cathod'eissatV potentials V'w, Vw and Vw, respectively.l In this'case the potential or contrast Vcontrol grid 76 israssumed to be*V somewhat higher than the potential of flood guncathode- 70, sayby- 50 volts"E -When an ,element of; storage surfacej80, comprising.V approximatelytwenty meshes of 'the storage screenstructure, ischarged-by the writing beam to the potential Vt, the eiective potentiallevel in passing through the intersticesbywayfof lines 98l is illustrated by plot 99a1of`;

Eig; 6.' V'Sincelthe potentiallevelof'this plot goestneg'ajtive 'with' respect to the potential VH of ood gun cathodek 70,l the electrons from flood gun 14 will not bei ableVv to pass through the interstices and lWilll b'e returned `to- V wards electrodes 56 and 6() and collector electrode 62'.v

vkWhen a storage surfaceelement is charged bythe writing beam to the sticking potential Vst, the effectivepotential levelfin passing through the interstices by way of lines 98 illustrated by curve 99 of Fig. 6; Assuming a certain velocityV distribution due Vto emission velocity spread and due to deviation from normalV incidence, the

f electrons Iofhigh -velocities in the forward direction will penetrate throughtheinterstices andproduce luminescens'e on viewinghscreen, 88.

When at storage' surface element ischarged bythe writing beam to the still higher potential-Vst, the eiective.

. potential level in passing through'l'interstices by waygof' lihes 98 is illustratedvby curve 99b'of Fig; 6. Inrthis case practicallyall" of the electrons fromV holding Agun- 14 Vdi'- rected toward this element will be lable to penetrate through storage `screen 78` thereby producing-the-brightest spotslin;..the picture;

4It is important-to note that with this arrangement, storage surface 80 remains always at a potential that is negative with respect to that of ood gun` cathode 70, so as to repel the electrons from flood gun 14. Therefore, the potential distribution imposed by the signal on storage surface 80 will not be disturbed by the continuous flow of electrons through the interstices. This potential distribution is illustrated by potential level curves 97h, 97, 97a, of Fig. 6, for the paths following line 96, Fig. 5 for the three conditions previously described in Fig. 4 wherein the potential of writing gun cathode takes on values Vw, Vw and Vw, respectively. As can be seen from Fig 6, the highest potential V"ss reached by a storage surface element is still negative with respect to the potential VH of flood gun cathode 70 and hence electrons will not impinge thereon.

The only charges, other than from writing gun 14, which may impinge on the storage surface are the positive ions produced by ionization which will drift towards the storage surface and tend to raise its potential. However, with suiciently good vacuum, it can be shown that the positive ion current will be suiciently low so that any positive charging due to the positive ions will not be detrimental. For example, if one assumes a pressure of -8 millimeter of mercury, the mean free path for electronswill be of the order of 108 millimeter. With an average electron path within the tube envelope of about 200 millimeters, it can be shown that the rate of change of potential on the storage screen will be of the order of 0.3 volt per second. This will not be detrimental if new signals are received a few times a second and the minimum signals are of the order of one volt.

Another embodiment of a storage tube capable of achieving the desired ideal presentation characteristic is illustrated in Fig. 7. The feature of this embodiment ofthe present invention is that uniformity of the sticking potential of the dielectric material deposited on the storage screen 78 is not essential, since the operation of the tube does not utilize this property of the dielectric. In this embodiment, a xed predetermined potential of zero volts is produced between the element of the storage surface and a writing screen when scanned by the writing beam. Hence, a charge distribution, in accordance with a signal, is accomplished by modulating the writing screen with the signal w-hile simultaneously scanning the writing beam over the writing screen.

The particular embodiment of the invention shown i Fig. 7 comprises an evacuated envelope 10, which in its left-hand portions, as viewed in the figure, has a writing gun 12 with appropriate electron beam deflecting means 16 together with a ood gun 14. The operation and construction of these units are the same as for thecorresponding units described for the storage tubeV illustrated in Fig. 1, except that in this case no signal is applied to intensity grid 2,4 and cathode 22 of writing gun 12, ood gun cathode 70 is grounded, and writing gun cathode 22 is maintained at a negative potential for the reasons hereinafter set forth. This negative potential is applied to cathode 22 froma direct-current potential source110, an appropriate value for the potential being -1000 volts with respect to ground. Intensity grid 24 is maintained negative with respect to writing gun cathode 22 by a connection through a lead 109 to a variable potential source 28. In the described mode of operation, no signal is applied to cathode 22 and hence no bypass capacitor around potential source 2S is required. The potential of variable potential source 28 is adjusted to produce the magnitude of writing beam current desired. Electrodes 56 and 60 and collector electrode 62 are maintained at the same potentials as specified for the tube illustrated in Fig. 1.

Facing the electron guns is a storage screen 78a which is similar to storage screen 78 illustrated in Figs. 1, 2 and `3, except that storage surface 80 is now placed on the side away from the electron guns. Metallic mesh support 76 of storage screen`78a is again used for contrast control, and will be referred to as contrast control grid 76. Contrast control grid 76 is biased with respect to the potential of ood gun cathode 70 by an amount equal to the maximum signal used which may be on the order of 10 volts. 'I'his potential is applied to contrast control grid 76 by a connection 103 to the negative terminal of a potential source 104.

An auxiliary conductive grid or metal mesh 102 is positioned adjacent to and behind storage screen 78a, with respect to the electron guns, mesh 102 serving to control the charge distribution produced on storage surface 80 by the writing beam. A video signal and a quiescent direct-current potential are applied to metal mesh 102, the signal being impressed at a terminal 107 through a capacitor 106, and the direct-current potential through a resistor 105 from a connection to the positive terminal of potential source 104. Capacitor 108 is used to bypass any signal around potential source 104. Since metal mesh 102 is used to write information on storage surface 80, it will be hereinafter referred to as the writing screen. The potential applied to writing screen 102 has a quiescent value that is positive with respect to the potential of contrast control grid 76y by an amount equal to approximately twice the maximum signal. The construction of Writing screen 102 may he the same as previously specified for contrast control grid 76 described in connection with the tube illustrated in Fig. 1.

Adjacent to and behind writing screen 102, with respect to the electron guns, in viewing screen 88 similar in construction to that used in the storage tube illustrated in Fig. l. A direct-current potential is applied to transparent conductive layer 84 of viewing screen S8 so that electrons passing through writing screen 1.02 will impinge on phosphor layer 82 with suicient energy to emit a satisfactory amount of light. An appropriate value for this potential is of the order of +10,000 volts with respect to ground, and is applied through a lead connected to the positive terminal of a potential source 101, the negative terminal of which is connected to ground.

As -described for the storage tube illustrated in Fig. 1, a high velocity writing beam is again used to deposit charges on storage surface S0, and a flood gun 14 provides slow electrons to penetrate through the interstices of the more positively charged elements of the storage screen. As previously mentioned, storage screen 78a is similar to storage screen 78 of Fig. l except that storage surface 80 is now placed on the side away from the electron guns.

In the operation of the tube illustrated in Fig. 7, writing screen 102 is scanned by the writing beam charging each element of storage surface 80 to the potential of writing screen 102 at the time the element ywas scanned. A source of flood electrons is 4furnished by flood gun 16. The charges on storagesurface 80 control the How of the flood electrons through the interstices of storage screen 78a by means of a gating action. The ood electrons then continue on through writing screen 102 to impinge on viewing screen 88. Since the flow of flood electrons through each element of storage screen 78a is controlled by the gating action produced by the charge thereon, the resulting image on `storage screen 38 corresponds to the charge distribution on storage surface 80 and hence is in accordance with the modulating signal.

To more completely explain the method of charging storage surface 80, reference is made to Fig. ll. In this figure, curve 113 represents the charging current to an element of storage surface 80 at the Itime of scanning by the writing beam. Potentials Vn, VE' land Vn" represent, respectively, potentials assumed byl writing screen 102 when modulated with a signal. In the case where the potential VE' of writing screen 102 is negative with respect to that of the corresponding storage surface element, the secondary electrons emitted from :the writing Vto Vnfanddesignated as Vc.

, 'ai Y `screen due "to bombardment by ithe -high -en'ergy electrons of ithe writing I'beam are attractedto `the more'positive storage surface Ielement lthereby reducing Vits potenti-al 'to `a potential Vc equal to the potential VnAof-the writing screen attheftime of lscanning.

{In Vthe case where -writing screen 102 is at a more positive potential, Vn, j-than the corresponding storage surface element, reflected electrons from the writing beam, as well as lhigh energy secondary yelectrons liber- -ated from writing'scrcen 1102*'oy the writing beam, will produce a secondaryA emission fratio `greater than unity from fthe storage surface element. Since 4the writing 'screen I102 is at -theVmore-positive (potential, the secon- -dary electrons liberated from Vvthe storage elementwwill be V4attracted i'tothe writing `screen "1102 thereby raising `the stan-taneous potentials, -VEYand Vn of writing screen.

1102, -virrespectiveo'f Ithe prior :charge on 4the storage surface element. Thepotential Vn represents the-quiescent -value ofpotential lfor 'the-writing Vscreen J102, the potential on 'the corresponding storage screen elemcn-tfbeingequal Referring to Fig. 8, there is illustrated the ,mannerin which contrast-control 4grid '76, -writing screen i102, vand liood gun cathode '70 should be .biased in order that the ilow of flood electrons from `llood gun 14 shall be controlled by `the Vstorage surface potential and not be inuenced appreciably -by the'signal voltage on writing screen 102. With the potential of flood gun cathode 70 grid v76 represented by Vn, the -potentials of storage survface elements represented by Vc, Vc `and Vc, corresponding, respectively, to instantaneous potentials, Vn', VE and Vn vassumed by writing screen 102 at the `time of bombardment by the writing beam, the potential levels are as illustratedinlig. 8.

i Referring to Fig. 8, line 2112 shows the potential 'level inproceedingthrough an openingofs'torage f'soreen-'78a along an electron path 111, the particular element containing the opening being charged at a time when Writing screen 102 was -at the quiescent value of potential Vn. As'the potential level along line '112 4'decreases until it is just equal to the potential VH ofllood -gun cathode 70 .and assuming a certain amount of spread Vin the velocities of the electrons emanating from flood gun 14, someelectrons will pass through -to writing screen 102 and vothers `will be turned back.

Line 1-12b shows the potential level in proceeding `through an opening of storage screen 78a along electron path 111, the particular element containing the opening being charged at a time when writing screen 102 was positive with respect to its lquiescent value of potential Vn. In this case, all uincident electrons on-the element will pass through the :openings as'there is no point that is negative with respect `to the potential of ood gun cathode '70. This condition is representative of a .bright spot .of the picture.

Referring still to Fig. 8, line `112a shows the potential profile in proceeding through-an opening of storage-screen 78a along electron path v111, the particular element containing the opening being charged at a time when writing screen 102 was negative with respect to its quiescent value of potential VE. Since the potential level in this instance goes negative with respect lto the ypotential of ood gun ,cathode 70, no electrons from ood gun 14 will pass .through the 'openings o'f this lelement. This condition is representative of a.dark spot [on therpicture.

.After passing through storage screen 17811, Very few electrons will impinge on the structureof writing screen 102, `but will jproceedon to the viewing screen 88. It i'sto` be understood that 'artificial velocity spread of elec- Vrepresented yby VH, the potential of contrast control 1 2 trous ffrrom holdinggun '14, as described -from the tube illustrated in Fig. l, can 'also be used with this storage tubefif desired. As the principle is the'same as'for'the previous tube, the description will not be repeated.

'-Inforder; to understand more Vfullytlle gating action of the storage screen meshes, reference is made to Figs. 9 and 10, wherein 4the equipoten-tial lines arerlabeled with respect vto the potential of Vflood vgun cathode' 70. In Fig. 9, equipotential lines are-sketched for an element of storage screen 78a for an open gate. The positive charge deposited on the storage surface element results ,in a positive leld penetration through the meshes of the element such that no equipotential lines of zero volts with respect to the potential of tlood gunpcathode extend completely across the openings of the element. Hence, as illustrated in YFig. n9, the primary electron emanatingfrorn the ilood'gun 114V will penetrate through the openings `of the storage yelement and continue on through. writing screen 1012 to impinge on viewingscreen 88, since at nort'irme was `its velocity in the forward'direetion reduced to zero, v

Illustrated 4in Fig. l0, equipotential lines referenced to the potential of-flood gun cathode 70 are sketched for aan. element of storagescreen t7th: 'for the closed gate action. Asshown in Fig. 10, the primary electron from flood gun 14 is repelled when it reaches the equipotential v away from the electron guns also insures low 'value of stray currents tostorage surface 80, Veither due to low veloeity electrons from flood vgun 14 or due to positive ions generated 1in vthefspace'between the electron guns and storage screen78a. The Voptimum bias voltages applied to 'contrast control `grid 76, `writing screen 102, and flood gun cathode, 70 `are somewhat dependent upon Vtheir actualdetailed geometry `including the thickness of the Vdielectric islands comprising istorage 'surface80, the .opening of the holes, the spacing between Athestorage surface 80 and the writingiscreen 102, the effective field penetration factor, n, ofthe writing screen 102, andthe strength .of the electric field between Awriting screen 102 and the viewing screen -88.

An alternative Inode of operation for the storage tube. illustrated in Fig. .7,'s shown in Fig. 112 wherein the potentials V", Vc and "Vc ofthe storage surface 80 as de-i. Scribedinll-Tig. 11are always negative Ywith respect to the -potential `offloodugun cathode70. v Thus, in this mode of operation, the primary electrons from flood .gun cathode 7.0 cannot reach the storage surface 80 and disturbthe potential distributionirnposed by the writing beam. "This mode .of operation can be .realized due to penetration of the electrostaticfelds through the interstices of the grids. Inithsmanner, eventhough thejgrids themselves are negative ^with respecttto thepotential of flood gun cathode 1'70, the potential in the openingsA of storage screen 78a can be made `somev'vhat positive .so that the electrons can vpenetrateithroughltothegrids.

Values o'fpotental for this mode of operation-will vary with thejtransparency and ,u ofthe particular grids used.- Representative values of potential withrespect to ground are v.altpproximate'ly as follows:

vPotential Vn .ofhoodigun :cathode 70:0 lvolts Potential Vnoffcontrastcontrol `grid 7.6=|20` volts Quiescent potential VE of writing screen 1`02=410vo1ts Referring to'Fig.'1'2, linesr114`b, 114,'114a representthe potential levels in lproceeding along electron `path 111 through .storage screen 78a and writing screen 102 to viewing screen "88 'for 'the situations where an element was charged to a potential V'fc, Vc, and Vc, respectively, corresponding to instantaneous potentials VE, VE, and Vn, respectively, of writing screen 102. More particularly, the potential level along line 114b is always :positive with respect to the potential of ood gun cathode 70. Accordingly, under these conditions, al1 primary electrons from `flood gun 14 travelling along path 111 will proceed on through to `viewing screen 8S resultingin a white spot in the presentation. Referring now to line 114, at one point on this curve the potential level is equal to potential VH of tlood gun cathode70. Under these conditions and due to velocity spread, only a portion of the electrons proceeding alongpath 111 will reach viewing screen Se, others beingrepelled, resulting in a gray spot in the presentation. On the other hand, line 114a shows the potential level for primary electrons attempting to traverse an opening in an element charged negative with respect to their source, hence all electrons will berepelled resulting in` a black spot in the presentation.

The principle of penetration of fields through the interstices of the storage screen 78a and writing screen 102 can also be used with the tubeof Fig. 7 together` with an auxiliary writing screen 116 for increasing the number of high energy electrons which in turn increase the rate of positive charging by the writing beam. A tube of this type is shown in Fig. 13.

Referring now to Fig. 13, there is shown a storageV tube similar to the storage tube illustrated in Fig. 7, except that auxiliary conductive grid or writing screen 116 is inserted between writing screen 102 and viewing screen 88 and the potentials appliedV to Ycontrast control grid 76 and writingscreen 162 are different. Contrast control grid 76 is vmaintained at a potential slightly positive with respect to the potential of flood gun cathode 70. This potential is impressed on contrast control grid 76 through a connection to the positive terminal of a potential source 11S, a suitable value for which may be volts with respect to the potential of ilood gun cathode 70. A quiescent value of potential of approximately volts negative with respect to that of contrast control grid 76 is impressed on writing screen 102 through resistor 105 from a terminal on a potential source 120. As before, signal voltages are impressed on terminal 107 through capacitor 106 to writing screen 102. A capacitor 10S bypasses signal voltages around potential source 120. Auxiliary writing screen 116 is maintained at a potential that is negative with respect to that of writing screen 102 by an amount greater than the critical potential for the dielectric material of storage surface 80. An appropriate value of this potential may be 200 volts negative with respect to writing screen 102.

In the operation of the tube of Fig. 13, in addition to secondary electrons being liberated by the high energy writing beam on the surface of writing screen 102, there will be additional secondary electrons liberated on the surface of auxiliary writing screen 116. If the potential of auxiliary writing screen 116 is maintained at a value below that of writing screen 102 by an amount more than the critical potential, the secondary electrons from aux-f. iliary writing screen 116 will arrive at the storage surface 30 with suiiicient energy to produce a net positive charging on storage surface 80 when the potential of the element on storage surface 80 is negative with respect to that of writing screen 102. The potential profile through the holes of storage screen 78a, writing screen 102, and auxiliary writing screen 116 will be similar to that shown in Fig. l2 where the field due to high voltage on viewing screen 88 penetrates through the holes and neutralizes the effect of negative biasing potential on auxiliary writing screen 116 if the iield penetration factor n of the screen is reasonably low.

It is especially pointed out that in each of the foregoing embodiments of the disclosed invention, the charge pattern on the storage screen is reset or converted by each -lil application4 of the signal, so that primary electrons from the flood gun produce a continuous image on the viewing screen in accordance with the charge pattern on the storage screen.

What is claimed is:

l. A direct-viewing electronic storage tube for converting an applied electrical signal into a visible image, said tube comprising a lluorescent viewing screen, a foraminous storage screen disposed coextensive with and adjacent said viewing screen, said storage screen including a plurality of secondary electron emissive storage elements on the side facing said viewing screen, a conductive grid interposed between said storage screen and said viewing screen, said conductive grid being adapted to receive said signal, means for producing a high energy electron beam of `elemental cross sectional area, means for successively directing said high energy electron beam` through said Storage screen to selected elemental areas `of said conductive grid in synchronism with said signal whereby the high energy electrons of said beam are both reflected from and produce secondary electrons from bombarded areas of said conductive grid to charge corresponding secondary emissive storage elements on said storage screen in register with said bombarded areas in either a positive or negative direction to the instantaneous potential of said conductive grid, and means for directing ood electrons through the interstices of said storage screen in proportion to the charge thereon and for directing them in a collimated beam through said conductive grid to said viewing screen to produce the image of said applied electrical signal.

2. The direct-viewing electronic storage tube as defined in claim l which additionally includes an auxiliary conductive grid interposed between said conductive grid and said Viewing screen to aid the chargingl of the storage elements of said storage screen in either a positive or negative direction to the instantaneous potential of said conductive grid.

3. A direct-viewing electronic storage tube comprising a fluorescent viewing screen; a storage screen disposed coextensive with and adjacent said viewing screen, said storage screen including a conductive mesh, and a layer of secondaryelectron emissive nonconductive dielectric material disposed uniformly over the side of said mesh facing said viewing screen to provide a ,storage surface; a conductive grid interposed between said storage screen and said viewing screen, said conductive grid having a secondary electron emissive surface; means applying an intelligence signal on said conductive screen; means for producing a high energy electron beam of' elemental cross sectional area; means for successively directing said high energy electron beam through said storage screen to selected elemental areas of said conductive grid in synchronism with said intelligence signal whereby the high energy electrons of said beam are both reilected from and produce secondary electrons from bombarded areas of the surface of said conductive grid to charge corresponding areas of said storage surface in register with said bombarded areas in either a positive or negative direction to the instantaneous potential of said conductive grid, thereby to produce a charge pattern representative of said intelligence signal on said storage surface; a source for developing ilood electrons; means maintaining said Source at a potential level that is positive: with respect to the potentials constituting saidcharge pattern; and means for directing said flood electrons through the interstices of said storage screen in proportion to the charge on said storage surface and in a collimated beam through said conductive grid to said viewing screen to produce a visual presentation of said intelligence signal.

4. The direct-viewing electronic storage tube as dened in claim 3 which additionally includes an auxiliary secondary electron emissive conductive grid interposed between said conductive grid and said viewing screen,

and' means for maintaining said a 'liary conductive grid e envelope on the other'side of saidlmesh for directing an el'eetronbeam of elemental cross-sectional area therethrough; saidy electron gun including a cathode; a writing screen disposed within said envelope `adjacent to and c'oexte'nsive with said'storage screen on said one side of said mesh for directing said electron beam back toward said storage elements; biasing means( electrically coupled t between said cathode and said storage screen for producing a potential diie'rence'between saidcathode and saidf storage screen, said potential difference being. greater than the critical potential of said storage elements; an r electrical deflection system cooperating withsaidlelectron beam for scanning said writing screen withl said electron b'e'al'n;` an.V electrical circuit coupled to said Ywriting screen for" varying'v the potentialV of4 said writing screen relative to said' storage screengpsa'id circuitbeing responsiveflto y the electrical signal `and` operable Ain synclr'onism'with saidielectrical deflection system for varying the potential of lsaid writingr screen in. accordance Vvviththe instanta- 16 Y .Y said electron beam charges each one ofsaid storage ele,- ments in register with elemental"a`reas of' said writing screen scanned by said electronibeam-to-a`potentialrcorrespondingr to the relative Vmagnitude'of' the electric-al signal 'at the instant said one element is beingfchar'ged by said electrony beam; a viewing screenwithin said en-V Velope and positioned on said one sidel of said mesh co- Y extensive therewith; flood gun means `disposed within said envelope on. said other side of said: mesh for directing llood electrons uniformly overfthe'are'aof saids'torage screen, the flood electrons passing through theinterstices of said storage screen in proportion to the charge thereon;

and means developing an electric potentialv and connected to said storage screen, said Writing screen,n and said viewing screen for directing said Hood electrons 'passing through said foramina to said viewing screen-in a col-v limatedV beam to produce a visual presentation of the charges on said storage elements. Y

6. The storage tube defined in claim 5, which further includes an auxiliary writing screen disposed within said envelopel between said writingl screen'tand said viewing screen and coextensive with and on said one side of' said mesh; and means for maintaining said auxiliary Writing nerousrelative magnitudes ofw the electrical signal, whereby screen at a potential that is negative with respect to the potential of said writing Vscreen by an1`amountr-greater than saidt critical potential' of said storage elements.

Referencesucited the le ofrthispatent UNITED STATES PATENTS4 

